The present invention relates to a hydraulic pressure controlling device for use in hydraulic brake systems of vehicles and, more particularly, to an improvement in deceleration sensing type braking hydraulic pressure controlling device which is disposed in a hydraulic circuit communicating a master cylinder and a rear wheel cylinder and adapted to control the hydraulic pressure in the rear wheel cylinder, upon sensing the deceleration of the vehicle when the latter is braked.
As a matter of theory, it has been known to those skilled in the art that, in order to obtain a stable braking through equalizing the decelerations of the front and rear wheels, it is necessary to gradually lower the rate of increase of the hydraulic pressure transmitted from the master cylinder to the rear wheel cylinder as compared with that for the front rear cylinder. For achieving such a hydraulic pressure control as faithfully as possible to the theory, a brake system incorporating a deceleration sensing type braking hydraulic pressure control valve, which is disposed in the hydraulic circuit between the master cylinder and the rear wheel cylinder, has been widely used.
This deceleration sensing type braking hydraulic pressure controlling valve has a valve chamber communicating the master cylinder and the rear wheel cylinder, a ball valve accommodated by the valve chamber in a floating manner, and a differential piston which is subjected to the pressures of both of the master cylinder and the rear wheel cylinder. In the braking, as the deceleration of the vehicle reaches a predetermined level, the ball valve is moved due to the inertia in the direction of movement of the vehicle, so as to close the valve opening to break the communication of the master cylinder and the rear wheel cylinder with each other. Once the rear wheel cylinder is isolated from the master cylinder, the pressure in the master cylinder is transmitted to the rear wheel cylinder through the action of the differential valve, so that the rate of pressure increase in the rear wheel cylinder is made smaller than that in the master cylinder.
Thus, the pressure in the rear wheel cylinder is increased at the same rate as the front wheel cylinder which is in direct communication with the master cylinder, before the valve opening is closed by the ball valve. However, after the valve opening is closed by the ball valve, the pressure in the rear wheel cylinder increases at a rate which is smaller than that in the front wheel cylinder. Consequently, the change of the braking force on the rear wheels is represented by two linear curves having different gradients and merging in each other at the instant at which the valve opening is closed by the ball valve. These linear curves approximates the theoretically required braking force distribution to some extent.
This deceleration sensing type braking hydraulic pressure controlling device, however, poses the following disadvantages.
Namely, the ball valve which is disposed in the flow passage of the braking fluid forwarded by the master cylinder is inevitably subjected to the dynamic pressure exerted by the flow of the braking fluid. This dynamic pressure inconveniently tends to displace the ball valve in the direction to close the valve opening. It is extremely difficult to make the ball valve standstill against this dynamic pressure.
In addition, the variation of the viscosity of the braking fluid attributable to the change of the working temperature causes a considerably large change in the dynamic pressure and, therefore, prevents the braking force distribution from approximating the theoretically required one.